There is known an electrocardiogram estimation apparatus which estimates an electrocardiogram signal indicating a change in electromotive force of heart by attaching electrodes to a surface of a human body and measuring a potential signal indicating a change in potential on the surface of the human body through the electrodes.
As illustrated in FIG. 1, the electrocardiogram signal includes waves having peaks called a P wave WP, a Q wave WQ, an R wave WR, an S wave WS, and a T wave WT at every beating. Time periods between the peaks of each wave in the electrocardiogram signal are used for diagnosis, test, or the like of disease. The heart rate is measured, for example, by acquiring the time period between the peaks of consecutive R waves WR.
It is known that, in two states where positions of the electrodes with respect to the heart are different from each other, even in the case where a change in electromotive force of heart is the same, the appearing shapes of the change in electromotive force of heart in the measured potential signals are different from each other. FIG. 2 illustrates two potential signals C10 and C11 measured in the two states where the positions of the electrodes with respect to the heart are different from each other even in the case where the change in electromotive force of heart is the same. For example, as illustrated in FIG. 2, the magnitude and timing of peak of each wave is changed according to the position of the electrode with respect to the heart.
Since a fetus is accommodated in a maternal body, it is difficult to attach the electrodes to a surface of the body of the fetus. Therefore, for example, an electrocardiogram estimation apparatus disclosed in Patent Literature measures a potential signal indicating a change in potential on a surface of a maternal body through electrodes which are attached on the surface of the maternal body and estimates an electrocardiogram signal of a fetus based on the measured potential signal.
In addition, there is known a fetal state estimation apparatus which estimates a fetal movement which is a movement of a fetus by generating an ultrasonic wave on a surface of a maternal body and observing the Doppler's effect of reflected waves obtained by reflection of the generated ultrasonic wave on the fetus in the maternal body (for example, refer to Patent Literature 2).                Patent Literature 1: JP 2006-204759 A        Patent Literature 2: JP 2013-505032 W        